While tracing an old Earth SOSdistress signal to its source, the USS Enterprise discovered this planet in 2266. A landing party from the Enterprisebeamed into a deserted, dilapidated street on the planet's surface. An initial survey of the planet, thereafter conducted by the landing party, revealed that the surviving population was composed of children who referred to themselves as the "Onlies". After discovering a cure for the virus and gaining the trust of the Onlies, the Enterprise arranged with Starfleet for teachers and advisors to be sent to this planet so they could aid the Onlies in rebuilding their civilization. (TOS: "Miri")

This planet is never given a name in the episode. In the final draft script of "Miri", however, it was repeatedly referred to as "Earth Two", including in the script sets list.

In the same script, this planet was initially described thus; "One green tinted planet in the center of the screen seems to pinpoint itself by its position and growth as THE planet [....] And what we see looks to be Earth... clearly... incredibly... an exact duplicate of our planet...." In the same script, the planet's mean density was said to be 5.552, rather than 5.517. North America was also identified in the script, and Hong Kong (or the planet's equivalent) was mentioned too. The street which the landing party beams into, when they first arrive on the planet, was described, in the scripted stage directions, as "what once was an important street in a great city."

In the original version of the episode, this planet lacked any cloud cover or weather patterns. In the 2006remastered version of "Miri", several new shots of the planet were incorporated into the episode, this time featuring more realistic atmospheric conditions and cloud cover.

According to the reference workStar Trek Maps (p. 31), the planet became known as "Miri's World". The planet orbited a yellow dwarfstar, which had the designation UFC 347601. Not only the planet, but also the entire solar system was an exact duplicate of the Sol system in every measurable detail. The planet had a population of thirteen million Onlies.

In his 1967first volume of Star Trek: The Original Seriesepisode adaptations, James Blish had no idea of the "duplicate Earth" premise of the television episode "Miri" (at the time of writing his first Star Trek adaptation book, he said that – probably because of the distance separation between those making Star Trek, and his residence in England at the time – he did not have access to the scripts that were used in shooting; he had only been given access to the original stories, which were often quite different from the TV shows). Blish instead wrote that Miri's planet was the fourth planet orbiting the star 70 Ophiucus. He described it as a beautiful Earth-like planet, having one large and two smaller continents connected by islands. The planet was located between twelve and fifteen light years from Earth and had been the first planet outside Earth's solar system to be colonized, in this case by refugees from the so-called "Cold Peace", about five hundred years before the events of the episode. These colonists were isolationists who violently repelled the first attempt to contact them by a later expedition from Earth, so no further contact was attempted. As it turned out, the 70 Ophiucus system was in a "backwater" part of the galaxy bypassed in subsequent years of Earth-based space exploration, and so the belligerent colony was easily ignored and almost forgotten. Around three hundred years before the events of "Miri", scientists on the planet developed the experimental life-prolongation project that resulted in the deaths of the adult population. Despite their close proximity, the distress signal sent by the colony failed to reach Earth because 70 Ophiucus stood between Earth and the center of the Milky Way, whose radiation created interstellar static that drowned out the SOS signal the colony had directed towards Earth.

According to the 2000 novel Preserver by William Shatner, the planet was called "Site 2713" and was revealed to have been duplicated from Earth on a subatomic level by the Preservers. It is one of three duplicates of Earth that have been discovered by 2375.